The Dahl salt sensitive (S) rat will be studied as an animal model of post-menopausal hypertension to investigate the mechanisms responsible for the hypertension. The pressor systems that will be evaluated are the renin-angiotensin system (RAS) and the sympathetic nervous system (SNS). The role of estrogen, aging, and salt intake as modulators of these pressor systems via their effects on the nitric oxide (NO) system will be investigated. The first specific hypothesis is that the aging process with the accompanying loss of estrogen activity is associated with a down regulation of the nitric oxide system resulting in hypertension. To test this hypothesis, Specific Aim 1 of this study is to monitor blood pressure and the level of activation of the NO system in intact, ovariectomized (OVX), and OVX+estrogen-treated Dahl salt sensitive and Dahl salt-resistant (R) female rats as they age from 3 month to 20-22 months of age. The second specific hypothesis is that the factors maintaining the hypertension associated with the loss of estrogen activity in Dahl S rats is determined by the level of salt intake. OVX performed at young, middle and old age will cause an increase in blood pressure, but the rise will be attenuated with increasing age. However, the level of blood pressure and the activity of the pressor systems contributing to hypertension will be higher pre-OVX because of the effects of aging on the RAS and SNS. Two specific aims will address this hypothesis. Specific Aim 2 will be to establish that the Dahl S elderly and OVX females maintained on low salt intake will become hypertensive as a result of an activation of the RAS. Estrogen administration will maintain activation of the NO system to suppress the RAS. Blockade of NO formation in low salt animals will increase RAS function, especially in the OVX+estrogen animals. Specific Aim 3 is to determine that high salt fed Dahl S elderly and OVX animals will develop a hypertension that is dependent on the activation of the SNS. As with the low salt animals, estrogen supplement will suppress the SNS stimulation through a NO-mediated mechanism. Together these studies will provide evidence that the arterial pressure of the Dahl S rat is sensitive to the removal of estrogen through OVX or aging suggesting a useful model of post-menopausal hypertension. By investigating the relationship of estrogen, NO and salt, important new information will be gained in the mechanisms whereby estrogen provides protection against hypertension. Importantly, a greater understanding will be obtained addressing why the protection disappears with the aging process.